The present invention relates to a pit prop. More particularly this invention concerns such a pit prop used in the face-working of a coal seam or the like.
A pit prop is known having a pair of longitudinally extending parallel feet which have front ends turned toward the face being worked and rear ends on which are pivoted a roof support or shield. A lift cylinder, normally of the hydraulic type, can raise the roof shield relative to the feet and press it tightly against the mine roof, simultaneously pressing the feet against the mine floor so as to lock the pit prop tightly in place. A support, normally a face conveyor, extends along the face in front of a row of such pit props and each pit prop has a central skid that is pivoted on this support and which is connected via a hydraulic slide ram to the feet. Thus the feet can be moved relative to the support when the lift cylinder is relaxed and the support can be moved relative to the feet when the shield is pressed against the ceiling.
Typically the floor-engaging feet of such a pit prop are of generally regular or uniform cross-section. These feet flank the above-mentioned skid which substantially fills the space between them. Thus material that falls down between the feet is compressed and cannot work its way backward of the prop into the back fill. Furthermore this compacted deposit which forms between the feet makes it relatively difficult to displace the pit prop toward the face as this face is cut away.
Furthermore this relatively limited play between the central skid and the two feet of the prop makes the entire roof prop assembly relatively rigid. In particular the stresses exerted on the skid must be taken up by the pivots on this skid. For this reason the various joints must be over-dimensioned in order to prevent breaking down of the arrangement.
In addition the relatively wide rear portions of the feet force the respective arms carrying the roof-engaging shield to pivot about axes which are fixed in these feet. Thus unevenness in the mine roof or torsion caused by twisting of the lower portion from forces exerted by the face conveyor can load these rear-end pivots considerably. Once again these pivots must be overdimensioned. Since the prop is used well under ground it is essential that it have a very long service life, as servicing it is almost impossible without removing the relatively bulky item completely from the mine.